a) Field of the Invention
The present invention relates to a finder optical system, and more specifically to a real image type finder optical system for single-lens reflex cameras.
b) Description of the Prior Art
Out of finder optical systems used in cameras and so on, a real image type finder optical system, which allows a photographer himself to confirm whether or not the optical system is in good focus, generally consists of an objective lens, a reticle having a diffusing surface and a focal plane, a condenser lens disposed in the vicinity of the reticle and an eyepiece lens. An example of configuration of a real image type finder optical system for singlelens reflex cameras is illustrated in FIG. 1, wherein the reference numeral 1 represents a photographic lens used as an objective lens of the finder optical system, the reference numeral 2 designates a quick return mirror which is removed from an optical path of the photographic lens 1 for allowing a light bundle to reach a film surface 3 for photographing, the reference numeral 4 denotes a reticle which has a focal plane having a light diffusing function and a Fresnel surface for converging the light bundle, the reference numeral 5 represents a condenser lens, the reference numeral 6 designates a pentagonal prism, and the reference numeral 7 denotes an eyepiece lens. In this finder optical system, a light bundle which is emitted from an object to be photographed and incident on the photographic lens 1 emerges from the photographic lens 1, and then is reflected by the quick return mirror 2 and imaged on the focal plane of the reticle 4. Further, the light bundle is converged by the condenser mirror 5, and the image formed on the focal plane of the reticle 4 is inverted by the pentagonal prism 6 is both the vertical and horizontal directions into an erect image, and reimaged by the eyepiece lens 7 onto a retina of an eye of a photographer.
In the finder optical system described above, the condenser lens 5 has two functions for converging the light bundle and correcting aberrations. First, description will be made of the converging function of the condenser lens 5. In FIG. 2 which illustrates a conceptional diagram of the real image type finder optical system, the light bundle which is emitted from the object to be photographed and transmits through the photographic lens 1 is diverged by the focal plane of the reticle 4 and then refracted as a whole by the condenser lens 5 toward the optical path as indicated by solid lines so that a portion of the light bundle is incident on a pupil of the photographer through the eyepiece lens 7. If the finder optical system does not use the condenser lens 5, on the other hand, the light bundle is diverged by the reticle 4, but light beams emitted from image points which are located at distances longer than a certain definite distance from the optical axis are not incident on the eyepiece lens 7 as indicated by a dashed line in FIG. 2, whereby the photographer cannot observe an image through the finder optical system. The condenser lens 5 has the function for converging the light bundle as described above.
Then, description will be made of the function of the condenser lens 5 for correcting aberrations. In the real image type finder optical system, aberrations such as spherical aberration can be corrected by the eyepiece lens 7 for allowing to observe a sharp image. Even when the aberrations are corrected by the eyepiece lens 7, however, only distortion can hardly be corrected since light beams coming from different image heights are overlapped with one another on the eyepiece lens 7. In order to correct the aberrations such as spherical aberration together with the distortion favorably by the eyepiece lens 7, this lens must inevitably have a very complicated composition, which is undesirable from viewpoints of manufacturing cost, size or form of finder body and so on. In contrast, the distortion can be corrected relatively easily by the condenser lens 5 on which the light beams coming from the different image heights are separated from one another.
Out of the two functions of the condenser lens 5 described above, the function for converging a light bundle can be allocated to a Fresnel lens. Speaking concretely, the thickness of the condenser lens 5 which is inevitably produced by imparting curvature to the condenser lens 5 can be reduced by using the Fresnel lens. Actually, there have already been offered for sale finders which adopt the reticle 4 having a diffusing surface 4a on one side and a Fresnel surface on the other side as shown in FIG. 3 for omitting the condenser lens 5, and therefore comprise reduced numbers of optical members and have compacter bodies. On the other hand, there have also been proposed finder optical systems in which curvature required only for correction of the aberrations is imparted to the lenses and the refractive power required for converging a light bundle is imparted to Fresnel lenses.
Further, there have already been disclosed reticles which have diffusing surfaces functioning to diffuse rays in directions varying from central portions toward marginal portions thereof for enlarging apertures of finder optical systems as exemplified by Japanese Patent Preliminary Publication No. Sho 59-189329.
In the case of the conventional finder optical system wherein only the condenser lens 5 is used without adopting the Fresnel lens, it is impossible to make finder body compact though the distortion can be corrected. In the case of the conventional finder optical system wherein the condenser lens 5 is used in combination with the Fresnel lens, thickness of the condenser lens 5 can be reduced to a certain degree, but it is impossible to effectively make the finder body compact since the finder body must contain a space for accommodating these two optical members. Further, in the case of the conventional finder optical system wherein the reticle 4 has a diffusing surface 4a and a Fresnel surface 4b which are formed on the object side and the eyepiece lens side respectively, steps formed on the Fresnel surface are visible when diopter of the photographer's eye is coincident with the steps on the Fresnel surface, thereby hindering observation of the image of the object to be photographed. This phenomenon is apt to occur particularly when the finder optical system is out of focus and forms a remarkably blurred image on the diffusing surface 4a. Furthermore, when the diffusing surface 4a is disposed on the side of the eyepiece lens and the Fresnel surface 4b is disposed on the object side, contrary to the case described above, the diopter of the photographer's eye is rarely coincident with the steps on the Fresnel surface 4b since the Fresnel surface is located on the object side of the diffusing surface 4a, but there is posed another problem that the location of the focal plane is varied dependently on thickness of the reticle. Even if the focal plane of the reticle 4 is attached to a camera body, the optical path is varied due to uniformity in thickness of the reticle 4 or variation of thickness of individual reticles, thereby producing direct influence on detecting accuracy of focused condition of the finder optical system. This influence on the detecting accuracy of the focused condition is extremely apt to be produced particularly in finder optical systems wherein the reticle 4 is exchangeable with another.